X-ray fluorescence analyzer with a plurality of measurement channels, and a method for performing x-ray fluorescence analysis

ABSTRACT

An X-ray fluorescence analyzer including an X-ray tube for emitting incident X-rays in the direction of a first optical axis. A slurry handling unit is configured to maintain a constant distance between a sample of slurry and the X-ray tube. A first crystal diffractor is located in a first direction from the slurry handling unit. The first crystal diffractor includes a first crystal and a first radiation detector configured to detect fluorescent X-rays diffracted by the first crystal at a first energy resolution. A second crystal diffractor is located in a second direction from the slurry handling unit. The second crystal diffractor includes a second crystal and a second radiation detector configured to detect fluorescent X-rays diffracted by the second crystal at a second energy resolution. The first crystal is a pyrolytic graphite crystal, the second crystal is of a material other than pyrolytic graphite, and the first and second crystal diffractors are configured to direct to their respective radiation detectors characteristic fluorescent radiation of a same element.

TECHNICAL FIELD

The invention relates to the technical field of X-ray fluorescenceanalysis. In particular the invention relates to the task of detectingrelatively small amounts of fluorescent radiation from an element ofinterest in the presence of significant background radiation and/orfluorescent radiation from other elements.

BACKGROUND

X-ray fluorescence analysis can be used to detect the presence andmeasure the concentration of elements of interest in a matrix of otherelements. For example in mining industry it is important to know,whether a mineral or metal of interest is present in a sample and inwhich quantities. In order to be applicable in an industrial process,the X-ray fluorescence analysis method should be reasonably accurateeven at relatively short exposure times, and possible to implement withrobust and mechanically reliable measurement devices.

A particular application of X-ray fluorescence analysis within themining industry is the analysis of elements of interest in slurries. Bydefinition, a slurry is a water-based suspension of fine, solidparticles of crushed and ground ore, in which the dry weight of thesolid particles is less than 90 per cent, typically 20-80 per cent, ofthe total mass of the sample. The fact that the sample is in the form ofslurry places particular requirements for sample handling. For example,it is advantageous to maintain the flow of the sample turbulent, so thatits constitution remains evenly mixed and the fractions do not separatefrom each other. At the same time the measurement geometry should remainas constant as possible in order not to cause unwanted geometry-basedvariations in measurement results.

The concentrations of elements of interest in the slurry are often verylow. For example copper, zinc, lead, and molybdenum need to be measuredin concentrations like 0.01 per cent or lower, and concentrations ofgold to be measured may be in the order of only some ppm, like 1-5 ppm.Such a low concentration makes the measurement difficult, because theintensity of fluorescent radiation from the element of interest is verylow, which inevitably increases the effect of statistical errors. Whenthe intensity is low in comparison to other radiation intensitiesinvolved, like fluorescent radiation from other, non-interestingelements, overlap with adjacent peaks causes problems. Measurement timescannot be made arbitrarily long, because the slurry comes as acontinuous flow from the refining process and is an important onlineindicator of what is happening in the process. The X-ray fluorescencemeasurement should be fast enough to detect trending changes in theslurry composition, so that the measurement results could be used tocontrol the refining process in real time.

SUMMARY

It is an objective of the invention to provide an apparatus and a methodfor performing accurate and reliable X-ray fluorescence analysis ofsmall concentrations of elements in slurry under demanding industrialconditions. Another objective of the invention is to provide such anapparatus and method that have the ability to adapt to different kindsof samples and different kinds of conditions.

The foregoing and other objectives are achieved by using at least twocrystal diffractors and their respective detectors that detectcharacteristic fluorescent radiation of the same element, so that one ofthe crystal diffractors contains a pyrolytic graphite crystal.

According to a first aspect, an X-ray fluorescence analyzer is provided.The X-ray fluorescence analyzer comprises an X-ray tube for emittingincident X-rays in the direction of a first optical axis, and a slurryhandling unit configured to maintain, in the direction of said firstoptical axis, a constant distance between a sample of slurry and saidX-ray tube. The X-ray fluorescence analyzer comprises a first crystaldiffractor located in a first direction from said slurry handling unit,said first crystal diffractor comprising a first crystal. The X-rayfluorescence analyzer comprises a first radiation detector configured todetect fluorescent X-rays diffracted by said first crystal at a firstenergy resolution. The X-ray fluorescence analyzer comprises a secondcrystal diffractor located in a second direction from said slurryhandling unit, said second crystal diffractor comprising a secondcrystal. The X-ray fluorescence analyzer comprises a second radiationdetector configured to detect fluorescent X-rays diffracted by saidsecond crystal at a second energy resolution. Said first crystal is apyrolytic graphite crystal, and said second crystal is of a materialother than pyrolytic graphite. Said first and second crystal diffractorsare configured to direct to their respective radiation detectorscharacteristic fluorescent radiation of a same element.

In a possible implementation of the first aspect, said second crystal isone of: a silicon dioxide crystal, a lithium fluoride crystal, anammonium dihydrogen phosphate crystal, a potassium hydrogen phthalatecrystal. This involves the advantage that sharp wavelength dispersivediffraction can be obtained with the second crystal.

In a possible implementation of the first aspect, said first energyresolution is better than 300 eV at a reference energy of 5.9 keV. Thisinvolves the advantage that the detector can provide accurate energydispersive detection within the relatively wide wavelength range passedby the pyrolytic graphite crystal.

In a possible implementation of the first aspect, said first radiationdetector is one of: a PIN diode detector, a silicon drift detector, agermanium detector, a germanium drift detector. This involves theadvantage that the first radiation detector may combine accurate andreliable operation with compact size and robust overall appearance.

In a possible implementation of the first aspect, said second radiationdetector is a gas-filled proportional counter. This involves theadvantage that relatively good detection efficiency can be achieved atrelatively low manufacturing cost.

In a possible implementation of the first aspect, said element is gold.This involves the advantage that even very low concentrations of arelatively valuable element can be detected.

In a possible implementation of the first aspect, said slurry handlingunit is configured to maintain a planar surface of said sample of slurryon a side facing said X-ray tube, said first optical axis is at anoblique angle against said planar surface, said first crystal diffractoris located at that rotational angle around said first optical axis atwhich said planar surface of said sample covers the largest portion of afield of view of the first crystal diffractor, and said second crystaldiffractor is located at another rotational angle around said firstoptical axis. This involves the advantage that fluorescent radiation canbe collected to the first crystal diffractor from as large spatial angleas possible.

In a possible implementation of the first aspect, said slurry handlingunit is configured to maintain a planar surface of said sample of slurryon a side facing said X-ray tube, and said first optical axis isperpendicular against said planar surface. This involves the advantagethat a number of measurement channels can be placed symmetrically aroundthe X-ray tube.

In a possible implementation of the first aspect, the input power ratingof said X-ray tube is at least 400 watts. This involves the advantagethat a relatively large amount of fluorescent radiation can begenerated.

In a possible implementation of the first aspect, the input power ratingof said X-ray tube is at least 1 kilowatt, preferably at least 2kilowatts, and more preferably at least 4 kilowatts. This involves theadvantage that an even larger amount of fluorescent radiation can begenerated.

In a possible implementation of the first aspect, the optical pathbetween said X-ray tube and said slurry handling unit is direct with nodiffractor therebetween. This involves the advantage that a largeproportion of the original incident radiation can be utilized, and theX-ray tube can be placed very close to the sample.

In a possible implementation of the first aspect, the X-ray tubecomprises an anode for generating said incident X-rays, and said slurryhandling unit is configured to maintain a shortest linear distance thatis shorter than 50 mm, preferably shorter than 40 mm, and morepreferably shorter than 30 mm between said sample of slurry and saidanode. This involves the advantage that a large proportion of theoriginal incident radiation can be utilized.

In a possible implementation of the first aspect, said X-ray tube is anX-ray tube of the end window type. This involves the advantage that ashort distance between X-ray tube and sample can be realized whilesimultaneously leaving ample space for detection channels.

In a possible implementation of the first aspect, the diffractivesurface of said pyrolytic graphite crystal is one of the following: asimply connected surface curved in one direction; a simply connectedsurface curved in two directions; a rotationally symmetric surface thatis not simply connected. This involves the advantage that the mostadvantageous form of the crystal can be selected for each application.

In a possible implementation of the first aspect, the X-ray fluorescenceanalyzer comprises an analyzer body, a front wall of said analyzer body,an opening in said front wall, and a holder for removably holding saidslurry handling unit against an outer side of said front wall andaligned with said opening in said front wall. This involves theadvantage that the slurry handling unit is easy to remove for servicing.

In a possible implementation of the first aspect, said X-ray tube andsaid first crystal diffractor are both inside said analyzer body, on thesame side of said front wall. This involves the advantage that thestructure is robust, and good protection can be obtained againstaccidentally irradiating anything.

In a possible implementation of the first aspect, the X-ray fluorescenceanalyzer comprises a filter plate on the optical path between said X-raytube and said slurry handling unit. This involves the advantage that thespectrum of the incident radiation can be tuned in a suitable way.

In a possible implementation of the first aspect, said filter plate islocated closer to said X-ray tube than to said slurry handling unit.This involves the advantage that the filter does not unnecessarilyobstruct the field of view of the detection channels.

In a possible implementation of the first aspect, the X-ray fluorescenceanalyzer comprises a calibrator plate and an actuator configured tocontrollably move said calibrator plate between at least two positions,of which a first position is not on the path of the incident X-rays anda second position is on the path of the incident X-rays and in a fieldof view of the first crystal diffractor. This involves the advantagethat calibrating can be easily automatized.

According to a second aspect, there is provided a method for performingX-ray fluorescence analysis. The method comprises irradiating a sampleof slurry with incident X-rays and receiving fluorescent X-rays from theirradiated sample; separating first and second predefined wavelengthranges from respective first and second portions of said receivedfluorescent X-rays with respective first and second crystal diffractors,wherein said first wavelength range and said second wavelength rangeboth include characteristic fluorescent radiation of a same element, andwherein said first wavelength range is at least twice as wide as saidsecond wavelength range; detecting the fluorescent X-rays in said firstand second separated wavelength ranges with respective first and secondradiation detectors, wherein the energy resolution of said firstradiation detector is better than 300 eV at a reference energy of 5.9keV, thus producing respective first and second detection results; andcalculating a concentration of said element in said sample from at leastone of said first and second detection results.

In a possible implementation of the second aspect, said calculatingcomprises calculating a combined intensity of background radiation andfluorescent X-rays from others than said element using at least one ofthe first and second detection results; subtracting, from the totalintensity detected in a wavelength range containing said characteristicpeak of fluorescent X-rays of an element to be measured in said sample,the calculated combined intensity of background radiation andfluorescent X-rays from other elements than said element of interest insaid sample; and providing the result of said subtracting as thecalculated intensity of said characteristic fluorescent X-ray peak. Thisinvolves the advantage that the accuracy of the measurement may beenhanced by using detection results from two detection channels.

In a possible implementation of the second aspect, said calculatingcomprises analyzing from said first and second detection results whetherthe influence of a characteristic peak from another element on the firstdetection result is larger than a predetermined threshold; if saidanalyzing shows that the influence of said characteristic peak from saidother element on the first detection result is larger than saidpredetermined threshold, calculating said concentration of said elementin said sample from said second detection result; and if said analyzingshows that the influence of said characteristic peak from said otherelement on the first detection result is not larger than saidpredetermined threshold, calculating said concentration of said elementin said sample from said first detection result. This involves theadvantage that the way of processing the results can be adapted to theactual occurrence of fluorescent radiation from other elements.

In a possible implementation of the second aspect, said element is gold.This involves the advantage that the presence and concentration of goldcan be detected despite the occurrence of another element with a nearbycharacteristic fluorescent peak.

In a possible implementation of the second aspect, said characteristicfluorescent radiation comprises a K- or L-peak of an element with30≤Z≤92, where Z is the atomic number of said element. This involves theadvantage that a large variety of elements can be detected.

In a possible implementation of the second aspect, said sample comprisessaid element within a matrix consisting of primarily elements with Z 8,where Z is the atomic number. This involves the advantage that forexample water-based slurries can be analyzed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and constitute a part of thisspecification, illustrate embodiments of the invention and together withthe description help to explain the principles of the invention. In thedrawings:

FIG. 1 illustrates a principle of X-ray fluorescence analysis in anindustrial process,

FIG. 2 illustrates a detail of an example of an X-ray fluorescenceanalyzer,

FIG. 3 illustrates an example of the use of a calibrator plate,

FIG. 4 illustrates an example of an X-ray fluorescence analyzer,

FIG. 5 illustrates some structural details of an example of an X-rayfluorescence analyzer,

FIG. 6 illustrates an example of a crystal diffractor,

FIG. 7 illustrates some geometrical aspects of a crystal diffractor,

FIG. 8 illustrates some shapes of diffractor crystals,

FIG. 9 illustrates an example of a radiation propagation geometry,

FIG. 10 illustrates another example of a radiation propagation geometry,

FIG. 11 illustrates an example of a radiation spectrum,

FIG. 12 illustrates another example of a radiation spectrum,

FIG. 13 illustrates another example of a radiation spectrum,

FIG. 14 illustrates another example of a radiation spectrum,

FIG. 15 illustrates a plurality of detection channels,

FIG. 16 illustrates example of radiation spectra,

FIG. 17 illustrates an example of a slurry handling unit,

FIG. 18 illustrates an X-ray tube with its optical axis perpendicularagainst the sample surface,

FIG. 19 illustrates an X-ray tube with its optical axis at an obliqueangle against the sample surface,

FIG. 20 illustrates an example of placing a plurality of detectionchannels,

FIG. 21 illustrates an example of placing a plurality of detectionchannels,

FIG. 22 illustrates measured detection accuracy of an example apparatus,

FIG. 23 illustrates measured detection accuracy of an example apparatus,

FIG. 24 illustrates measured detection accuracy of an example apparatus,and

FIG. 25 illustrates measured detection accuracy of an example apparatus.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of the principle of using an X-rayfluorescence analyzer in an industrial process. It is typical toindustrial processes that the sample to be analyzed may come as a moreor less continuous flow of sample material, so that there is a samplehandling unit or system that brings the sample to the analysis and takesit away after the analysis. In the schematic illustration of FIG. 1 thesample 101 comes as a flow of sample material on a conveyor 102, whichhere constitutes the sample handling system. An X-ray source 103generates a beam 104 of incident X-rays that hit a portion of the sample101 that is within the field of view of the beam 104. Fluorescent X-rays105 are emitted in all directions, and some of them are collected to adetection system that in FIG. 1 comprises a first slit 106, awavelength-dispersive diffractor crystal 107, a second slit 108, and aradiation detector 109. The plant may comprise a control computer system110 that may control the control subsystems 111 and 112 of the conveyor102 and the X-ray fluorescence analyzer 113 respectively.

The generation of fluorescent X-rays is a stochastic process by nature,so any analysis that is performed on the basis of received fluorescentX-ray photons is basically the more reliable, the more such photons canbe collected. A known way to increase the statistical reliability of anX-ray fluorescence analysis is to lengthen the duration of time that thesample remains illuminated by the incident radiation. If the sample isstationary, this means simply waiting a longer time before the sample ischanged. The nature of an industrial process may require however thatthe sample comes as a constantly moving stream. Even then the concept ofa longer measurement time exists in a way, because if the constitutionof the sample stream remains essentially constant, accumulating theamounts of detected fluorescent X-ray photons from the moving samplestream for X minutes is essentially the same as keeping a portion of thesample material stationary in the analysis for X minutes.

There are limits, however, to how long the averaging time may be when aconstantly moving sample stream is analyzed, because the constitution ofthe sample stream does change, and these changes may be important andshould therefore be noticed. Additionally if the sample comes in theform of a slurry there are other factors that make the situation morecomplicated, like the requirement that the flow of the slurry shouldremain turbulent in order to prevent separation of the solid and liquidphases. It is not uncommon that a sample of slurry flows through theslurry handling unit at a rate in the order of 20 liters per minute. Anobjective of the invention is that reasonably good detection resultscould be obtained by using averaging times in the order of minutes, like2 minutes or like 3 to 5 minutes.

In the following, improvements to the X-ray fluorescence analysisthrough factors like measurement geometry, incident radiation power,selection of diffractor crystal materials, selection of detector types,use of a plurality of detection channels, and advanced utilization ofdetection results, are therefore considered.

FIG. 2 is a schematic cross section of certain parts of an X-rayfluorescence analyzer. The X-ray fluorescence analyzer comprises anX-ray tube for emitting incident X-rays 206 in the direction of a firstoptical axis 204. A radiation window 203 of the X-ray tube is seen inFIG. 2. For handling a sample 202 of slurry the X-ray fluorescenceanalyzer comprises a slurry handling unit 201, which in this casecomprises a sample chamber 208 or sample cell equipped with inlet andoutlet connections. The exact way in which the sample chamber 208 andits inlet and outlet connections are formed in order to ensure aturbulent flow of the sample 202 inside the chamber is not pertinent tothis particular description. As an example, principles explained in theinternational patent application published as WO2017140938 may befollowed. In any case, the slurry handling unit is configured tomaintain a constant distance between the sample 202 of slurry and theX-ray tube. The constant distance may be considered for example in thedirection of the first optical axis 204.

Keeping the distance constant has the effect that the measurementgeometry does not change, at least not with reference to the distanceand viewing angle that have an important effect on what proportion ofthe incident X-rays 206 will hit the sample 202. As such, the apparatusmay comprise means for changing the distance, for example by changing adistance at which the X-ray tube is installed. In other words, it is notmandatory that said distance will always remain the same. Merely, it isadvantageous for the purposes of straightforward processing of thedetection results that the mechanical configuration of the X-rayfluorescence analyzer allows maintaining said distance constant during ameasurement, whenever wanted.

FIG. 3 illustrates how the slurry handling unit 201 comprises a samplewindow 301 in a wall of the sample chamber 208 for allowing X-rays topass through while keeping the sample 202 of slurry within said samplechamber 208. The sample window 301 is an opening covered by a windowfoil 302 made of a material that is as transparent to X-rays aspossible, while simultaneously being strong enough mechanically towithstand the pressure of, and mechanical wear caused by, the flowingslurry. This way the slurry handling unit is configured to maintain aplanar surface of the sample 202 of slurry on a side facing the X-raytube. In the geometry shown in FIGS. 2 and 3 the first optical axis 204is perpendicular against said planar surface.

Also shown in FIGS. 2 and 3 is a front wall 303 of an analyzer body, andan opening in said front wall 303. Another window foil 304 covers saidopening in the front wall 303. Just like the window foil 302 of thesample window 301 in the sample chamber 208, the other window foil 304is made of a material that is as transparent to X-rays as possible. Thepurpose of the other window foil 304 is to protect the inside of theX-ray fluorescence analyzer device against dust, moisture, and othercontaminants that may be abundant in its surroundings in an industrialprocess.

FIG. 2 shows how the incident X-rays 206 that hit the sample 202 giverise to fluorescent X-rays 207. These are originally directed to alldirections, but of interest are those fluorescent X-rays 207 that comeout of the sample chamber 208 through the sample window 301 and can becollected to one or more detection channels. The location, geometry, andproperties of such detection channels are described in more detaillater.

Another feature shown in FIGS. 2 and 3 is a filter plate 205 on theoptical path between the X-ray tube and the slurry handling unit. Afilter plate of this kind is an optional feature. It works as ahigh-pass filter by attenuating particularly the lowest-energy portionof the X-rays that were originally generated in the X-ray tube. Thematerial and thickness of a filter plate 205, if one is used, can beselected so that it passes those X-rays that are energetic enough togenerate fluorescence in the element(s) of interest in the sample 202.It is particularly useful to use a filter of the high-pass type insteadof e.g. a primary diffractor that would constitute a band-pass filter,because the high-pass filter will pass a wide range of more energeticincident X-rays, which are then available for generating fluorescentX-rays in a number of elements of interest simultaneously.

If a filter plate 205 is used, it is advantageous to place it closer tothe X-ray tube than to the slurry handling unit. The filter plate 205can be even attached to the X-ray tube, so that it is very close to theradiation window 203 of the X-ray tube. If the filter pate 205 isadditionally dimensioned in the transverse direction so that it is onlylittle larger, or not larger at all, than the radiation window 203, itcan be ensured that the filter plate 205 does not unnecessarily coverany of the field of view that would otherwise be available for thedetection channels. The thickness of the filter plate 205 may be in theorder of a millimeter or even less, so the use of a filter plate doesnot increase the overall distance between the X-ray tube and the sampleto any significant extent.

Another feature that is shown in FIGS. 2 and 3 is a calibrator plate 305that can be controllably and selectively brought into a position inwhich it is on the path of the incident X-rays 206 and in a field ofview of the detection channels that are used to receive the fluorescentX-rays 207. A calibrator plate 305 has a very exactly known composition,so it can be used to calibrate the detection channels from time to time.If the calibration process should be automatized, the X-ray fluorescenceanalyzer may be equipped with an actuator that is configured tocontrollably move the calibrator plate 305 between at least twopositions, one of which is the position shown in FIG. 3 and the other isa position that is not on the path of the incident X-rays 206.

FIG. 4 illustrates an example of an X-ray fluorescence analyzeraccording to an embodiment of the invention. It comprises an analyzerbody 401 that acts as the basic support and protective structure. Thefront wall 303 of the analyzer body is visible on the left in FIG. 4. Asexplained earlier with reference to FIGS. 2 and 3, there is an openingin the front wall 303 for the incident X-rays generated by an X-ray tube402 to pass through. A holder 403 is provided for holding the slurryhandling unit 201 against an outer side of the front wall 303, alignedwith said opening in the front wall 303.

In an advantageous embodiment the holder 403 may be configured to holdthe slurry handling unit 201 removably against the front wall. Theholder 403 may comprise for example hinges that allow turning the slurryhandling unit 201 to the side, or a bayonet mount that allows quicklydetaching the slurry handling unit 201 from the front wall 303, so thatthe window foils described above with reference to FIGS. 2 and 3 areexposed. This allows relatively straightforward inspecting and servicingof those parts that are critical for the propagation of both theincident X-rays and fluorescent X-rays. The solid particles in theslurry may cause significant wear to the inside of the window foil 302of the sample window 301 (see FIG. 3), so it is advantageous to equipthe sample window 301 with a mechanism that allows replacing the windowfoil 302 when necessary.

A portion of the X-ray fluorescence analyzer that is marked with adashed rectangle 404 in FIG. 4 is shown from the direction of theoptical axis of the X-ray tube 402 in FIG. 5. This illustration shows anexample of how an actuator 501 can be provided for controllably movingthe calibrator plate 305 between the two positions. In the firstposition, which is shown in FIG. 5, the calibrator plate 305 is not onthe path of the incident X-rays that come out of the radiation window203 of the X-ray tube. In the second position the calibrator plate 305would be essentially concentric with the radiation window 203 in FIG. 5.

FIGS. 4 and 5 also show how one or more detection channels 502 may beprovided. The structure and operation of a detection channel will bedescribed in more detail later in this text. FIGS. 4 and 5 illustrate apositioning principle, according to which each of the detection channelsis located at a respective rotation angle around the optical axis of theX-ray tube 402. When the optical axis of the X-ray tube 402 isperpendicular against the planar surface of the sample (which is definedby the sample window that is a part of the slurry handling unit 201),this way of placing the detection channels allows arranging an exactlyequal field of view for all detection channels.

Other features shown in FIG. 4 are the provision of electronics boxes405, 406, and 407 inside the analyzer body 401 for each of the detectionchannels and for the X-ray tube 402, as well as the provision of acooling water circulation 408 for the X-ray tube 402.

FIG. 6 is a schematic illustration of certain parts of what was called adetection channel above. Major features of the detection channel of FIG.6 are a crystal diffractor 601 and a radiation detector 602. As its nameindicates, the crystal diffractor 601 comprises a crystal 603, which maybe called the diffractor crystal or just crystal for short. The crystal603 is the wavelength-dispersive component of the crystal diffractor601. A first slit 604 may be provided on a first optical path 605between the slurry handling unit (not shown in FIG. 6) and the crystal603, and a second slit 606 may be provided on a second optical path 607between the crystal 603 and the radiation detector 602. Since thediffractive properties of the crystal 603 for X-rays are highlydependent on wavelength of the X-rays, this kind of an arrangement canbe used to separate a particular wavelength range from that portion ofthe fluorescent X-rays that were originally emitted into that directionin which this particular crystal diffractor is located. Referencedesignator 608 illustrates a casing that encloses the crystal diffractor601, offering structural support for all of its components.

FIG. 7 illustrates an example of a crystal diffractor in an axonometricprojection. The crystal diffractor is thought to be located in a firstdirection from a slurry handling unit (not shown in FIG. 7), so that thefirst optical path 605 represents the nominal direction of that portionof fluorescent X-rays that are received in this crystal diffractor. Thefirst 604 and second 606 slits are formed between the respective limiterpieces, and the second optical path 607 represents the nominal directionof the diffracted fluorescent X-rays that are directed to the radiationdetector (not shown in FIG. 7). The crystal diffractor is enclosed in acasing 608 delimited by a first planar surface 701 and a second planarsurface 702 that is parallel to said first planar surface 701.

The mechanical structure described here is advantageous, because theplanar surfaces 701 and 702 offer a support to which the internal partsof the crystal diffractor can be attached in a relatively simple way.

Diffraction of X-rays in a crystal is governed by Bragg's law, whichties the diffraction angle to the distance between reticular planes.Conventional crystal diffractors have used crystals of e.g. silicondioxide, lithium fluoride, ammonium dihydrogen phosphate, or potassiumhydrogen phthalate, because sufficiently large monocrystalline pieces ofthese materials can be manufactured relatively easily at the requiredaccuracy in the desired shapes. However, it has been found that whilethe wavelength selectivity of such conventional crystals is relativelygood, the efficiency at which incoming X-rays are diffracted isrelatively poor.

Pyrolytic graphite is an alternative material for producing the crystalfor a crystal diffractor. Pyrolytic graphite is a general term thatrefers to materials that were manufactured from organic compoundscontaining planar structures like benzene rings, by subjecting them tohigh temperatures, causing essentially only the carbon atoms of thestructure to remain. The original planar molecular structures cause thepyrolytic graphite to exhibit a highly ordered microscopic structure,for which reason it is often referred to as HOPG (highly orientedpyrolytic graphite) or HAPG, in which the latter refers to a slightlydifferent method of synthesizing the material. Pyrolytic graphite isoften not monocrystalline in the same sense as the more conventionalcrystal materials mentioned above, but polycrystalline. In order tomaintain consistency with the established wording on this technicalfield it is nevertheless practical to refer to the diffractor elementmade of pyrolytic graphite as the “crystal”. In the followingdescription the term “HOPG crystal” will be used.

The efficiency of a HOPG crystal as a diffractor of fluorescent X-rayshas been found to be significantly better than that of the conventionalmaterials of diffractor crystals. In other words, a significantly higherproportion of X-rays that hit a HOPG crystal are actually diffractedthan with the conventional crystal materials. However, thepolycrystalline nature of pyrolytic graphite means that not allreticular planes are as exactly oriented as in e.g. monocrystallinesilicone dioxide. This in turn means that the wavelength selectivity ofa HOPG crystal in a crystal diffractor is not very exact: fluorescentX-rays that get diffracted into a particular direction represent a rangeof wavelengths around the nominal wavelength that according to Bragg'slaw would be diffracted into that direction, and this range issignificantly wider than in X-rays diffracted by the conventionalcrystal materials.

The less accurate wavelength selectivity of the HOPG crystal is not,however, a serious drawback because it can be balanced with otherfactors in the design of the X-ray fluorescence analyzer. One possibleapproach is to use a solid-state semiconductor detector as the radiationdetector 602 to which the fluorescent X-rays in the separated wavelengthrange are directed from the HOPG crystal. The radiation detector 602 maybe for example a PIN diode detector, a silicon drift detector, agermanium detector, or a germanium drift detector. Contrary to forexample gas-filled proportional counters, the energy resolution ofsolid-state semiconductor detectors can be made more accurate. It iscustomary to express the energy resolution of a detector of X-rays at areference energy of 5.9 keV. A solid-state semiconductor detector of thekind mentioned above may have an energy resolution better than 300 eV atsaid reference energy of 5.9 keV.

Combining the use of a HOPG crystal in the crystal diffractor 601 to theuse of a solid-state semiconductor detector as the radiation detector602 may result in an advantageous situation in which the crystaldiffractor 601 is configured to separate a predefined first wavelengthrange from fluorescent X-rays 207 that propagate into the direction atwhich the crystal diffractor 601 is located (with reference to theslurry handling unit 201), and configured to direct the fluorescentX-rays in the separated predefined first wavelength range to theradiation detector 602 that is a solid-state semiconductor detector. Thegood energy resolution of the solid-state semiconductor detector is thenused to produce a measurement resuit that indicates an energy spectrumof the fluorescent X-rays in the separated predefined first wavelengthrange. From said energy spectrum, and possibly using other measurementsas support, the amount of fluorescent X-rays from the element ofinterest can be determined with relatively good accuracy.

The geometrical form of the diffractive surface of the HOPG crystal isanother factor to consider in the design of the X-ray fluorescenceanalyzer. FIG. 8 illustrates some examples of geometrical forms. Here itmay be noted that exactly speaking the “crystal” is only a thin layer ofcrystalline (monocrystalline, as in the case of silicon dioxide forexample, or polycrystalline, as in the case of HOPG, for example)material that constitutes the actual diffractive surface. The crystaldiffractor comprises a substrate to which the crystalline material isattached. Examples of substrate materials are for example glass andaluminum, but the substrate material could also be iron or any othersuch material that is not prone to causing unwanted, interferingfluorescent radiation by itself. The crystalline material may beattached to an appropriately formed surface of the substrate through forexample Van der Waals forces. Alternatively the crystalline materialcould be grown directly upon the appropriately formed surface of thesubstrate, or some other suitable attachment method like glue could beused.

Together the substrate and the crystalline material constitute athree-dimensional entity, and examples of these entities are seen inFIG. 8. In order to maintain consistency with the established parlanceon this technical field, these entities are called crystals in this textdespite of the slight inaccuracy of this term that is explained above.The term diffractive surface refers to the external, exposed surface ofthe crystalline material at which the diffraction of X-rays takes place;strictly speaking the diffraction of X-rays takes place at the reticularplanes inside the crystalline material close to the surface that is herecalled the diffractive surface.

A feature common to crystal 603, crystal 802, and 804 in FIG. 8 is thata three-dimensional geometrical shape of the entity constituted by theHOPG crystal and the substrate is that of a prism, one side face ofwhich is cut away by the curved diffractive surface. The imaginary formof the prism is shown with dashed lines in the upper-line illustrationsof these three crystals.

The lower-line illustrations of the same crystals in FIG. 8 shows howthe way in which the diffractive surface is curved is different in allthree cases. In crystal 603 the diffractive surface 801 is curved in onedirection (longitudinal direction) only. In other words, if an imaginarytransverse line was drawn across the diffractive surface 801 at anylocation, like the dashed line shown in FIG. 8 for example, it wouldalways be straight. A particular advantage of this kind of a crystal isthat it is relatively easy to manufacture. Comparing to FIGS. 6 and 7 itcan be seen that the radius of curvature of the diffractive surface 801lies in a plane defined by the first 605 and second 606 optical paths.This plane is also parallel to the planar surfaces 701 and 702.

In crystal 802 the diffractive surface 803 is curved in two directions(longitudinal and transverse), forming a part of a toroidal surface.This means that if a transverse arc was drawn across the diffractivesurface 803 at any location, like the two dashed arcs shown in FIG. 8for example, each of these transverse arcs would be identical to eachother. Although this geometrical form may be somewhat more complicatedto manufacture at the required accuracy than that of surface 801 on theleft, it involves the advantage that it focuses the diffracted X-raysmore accurately.

In crystal 804 the diffractive surface 805 is curved in two directions(longitudinal and transverse), but in a different way than surface 803in the middle. The diffractive surface 805 forms a part of arotationally symmetric surface, the rotational axis 806 of which is inthe plane defined by the optical paths of the incoming and diffractedX-rays. This means that if a transverse arc was drawn across thediffractive surface 805, like the dashed arc in FIG. 8 for example, theradius of curvature of such a transverse arc would be differentdepending on at which longitudinal location it was drawn. In FIG. 8 itcan be seen that the dashed arc in the middle is not as pronouncedlycurved as the arc-formed edges seen at the ends of the crystal 804. Thisis because the dashed arc is located further away from the rotationalaxis 806 than the arc-formed edges at the ends of the crystal.

Mathematically speaking, a rotationally symmetric surface is formed whena continuous curve is rotated about the rotational axis. The form ofsaid continuous curve defines, how far from the rotational axis eachpoint of the surface will be, and what properties the surface may have.One example of a curve that could be used to form the diffractivesurface 805 in FIG. 8 is a section of a logarithmic spiral. Althoughthis kind of a surface is more complicated to manufacture than thoseintroduced above as surfaces 801 and 803, a rotationally symmetricsurface made with a section of a logarithmic spiral involves theinherent advantage that it provides very accurate focusing of diffractedX-rays.

A feature that is common to all diffractive surfaces 801, 803, and 805in FIG. 8 is that in topological sense they are simply connectedsurfaces. A simply connected surface is one that is path-connected (i.e.any two points on the surface can be connected with a path that belongswholly to said surface), and additionally any loop-formed path can becontinuously contracted to a point so that also all intermediate formsof the contracted loop belong wholly to said surface.

An intuitive description of a simply connected surface is that it doesnot have holes. As such, it could be possible to drill a small holethrough any of the diffractive surfaces 801, 803, or 805 in FIG. 8without changing their properties as diffractors more than just bydecreasing the surface area by the amount that was drilled away. Forthis reason it is defined here that the requirement of the surface beingsimply connected in topological sense is to be interpreted to concernthe general form of the surface: under such an interpretation a smallhole in the surface does not yet mean that it would not be simplyconnected. Another definition of how the requirement of being simplyconnected should be interpreted is as follows: if the crystal is “lyingon its side” as in FIG. 8 (i.e. a main radius of curvature, whichdefines the longitudinal curvature between the ends of the crystal, isin a horizontal plane; so that the diffractive surface is generallyvertically oriented), any imaginary horizontal line would pierce thediffractive surface at one point at the most. A surface is a simplyconnected if it fits at least one of these intuitive descriptions. Onthe right in FIG. 8 a crystal 807 is shown as a comparative example. Thediffractive surface 808 of the crystal 807 is curved in two directions(longitudinal and transverse), forming a complete rotationally symmetricsurface, the rotational axis 809 of which could be in a plane defined bythe optical axes of the incoming and diffracted X-rays. The curve, therotation of which about the rotational axis 809 defined the form of thediffractive surface 808, may be for example a section of a logarithmicspiral. It is obvious that the diffractive surface 808 is not simplyconnected in topological sense, because no closed curve thatcircumnavigates the bore of the surface can be contracted to a point.Crystals of this kind are relatively complicated to manufacture, butthey can be used, together with suitable shields (not shown in FIG. 8)that block the propagation of direct, not diffracted X-rays, to collectfluorescent radiation from a larger spatial angle than those with asimply connected surface like 801, 803, or 805.

The geometric shape and the resulting optical properties of thediffractive surface may have an effect on how other parts of the crystaldiffractor should be designed. Above it was explained how the crystaldiffractor 601 may comprise a first slit 604 on the first optical path605 between the slurry handling unit 201 and the (pyrolytic graphite)crystal, and how there is the second optical path 607 between the(pyrolytic graphite) crystal and the radiation detector 602. If thediffractive surface 801 of said (pyrolytic graphite) crystal 603 iscurved in one direction only, with a radius of curvature in a planedefined by said first 605 and second 607 optical paths, it isadvantageous to make said first slit 604 a linear slit orientedperpendicular against said plane, like in FIG. 7. If the diffractivesurface 803 of said (pyrolytic graphite) crystal 802 is curved in twodirections, forming a part of a toroidal surface, it is advantageous tomake said first slit a curved slit with a radius of curvature orientedperpendicular against said first optical path. If the diffractivesurface 805 of said (pyrolytic graphite) crystal 804 is curved in twodirections, forming a part of a rotationally symmetric surface, therotational axis 806 of which is in the plane defined by said first andsecond optical paths, it is advantageous to make said first slitpoint-like.

If a second slit 606 is used on the second optical path 607, similarconsiderations may apply. However, it should be noted that the secondslit is not always necessary: its use is related to attenuatingbackground and scattered radiation particularly with diffractor crystalsthat are highly wavelength-selective. Taken that the wavelengthselectivity of a HOPG is not that high, the additional advantage gainedwith a second slit is relatively small.

If a second slit is used on the second optical path 607 between the(pyrolytic graphite) crystal 603, 802, 804 and the first radiationdetector, the geometry of the crystal diffractor may follow for examplethe principle of a Johann geometry or a Johansson geometry. These areillustrated in FIGS. 9 and 10 respectively. In FIG. 9 a center point 902of said diffractive surface, said first slit 604, and said second slit606 are located on a Rowland circle the radius of which is R. A radiusof curvature of said diffractive surface in the plane defined by saidfirst and second optical paths is 2R, and a radius of curvature ofreticular planes 901 in said crystal is 2R. This means that the firstcrystal diffractor has a Johann geometry. In FIG. 10 a center point 1002of said diffractive surface, said first slit 604, and said second slit606 are similarly located on a Rowland circle the radius of which is R.However, here a radius of curvature of said diffractive surface in theplane defined by said first and second optical paths is R, and theradius of curvature of reticular planes 1001 in said crystal is 2R, sothat the first crystal diffractor has a Johansson geometry.

In order to maintain a compact size of the crystal diffractor it isadvantageous if the magnitude of R can be kept relatively small. As anexample, R may be at most 40 centimeters.

FIGS. 11 to 14 are schematic illustrations of spectra of fluorescentX-rays in certain cases. The spectra are typically expressed as detectedcounts at each photon energy. In practice the detector that produces thecounts has a certain energy resolution that defines, how close to eachother the energies of two photons may be so that the detector is capableof producing two different kinds of output signals. Signal processing isused to classify the received X-ray photons into energy bins of finitewidth, and the counts are given per energy bin. The more accurate thedetector resolution, the narrower (in terms of energy units) the energybins can be made.

In FIG. 11 the graph 1101 is smooth without any visible peaks orspectral holes. Such a spectrum is rarely obtained in practice, but itillustrates a situation in which only background and randomly scatteredradiation is received, without any characteristic peaks of elements ofinterest. In FIG. 12 the graph 1201 is otherwise the same, but there isa characteristic peak 1202 of an element of interest. The problem isthat the concentration of the element of interest in the measured sampleis so small that the height of the characteristic peak 1202 is low withrespect to the general level of the spectrum at the same energy range.Thus even if a relatively large number of photons are observed in thatenergy range, relatively few of them are actually fluorescent photonsfrom the element of interest.

The energy of a photon is inversely proportional to its wavelength, sowhen the wavelength selectivity of various diffractive crystals has beenconsidered above, energy selectivity could be considered quite as well.FIG. 13 illustrates schematically what the radiation detector of acrystal diffractor equipped with a HOPG crystal could receive. Theenergy range 1301 of fluorescent X-rays that the HOPG crystal woulddirect to said radiation detector is relatively wide, which is a directresult of the relatively modest wavelength selectivity of the HOPGcrystal. At the same time, however, the diffraction efficiency of theHOPG crystal is relatively good. Thus the radiation detector wouldreceive a significant proportion of the photons falling within the twohatched areas in FIG. 13. Of these, the photons belonging to the firsthatched area 1302 are background and scattered photons, while thephotons belonging to the second hatched area 1303 are actual fluorescentphotons from the element of interest.

FIG. 14 illustrates schematically what the radiation detector of acrystal diffractor equipped with a silicon dioxide (or otherconventional) crystal could receive in the same situation. The energyrange 1401 of fluorescent X-rays that the conventional crystal woulddirect to its radiation detector is relatively narrow, which is a directresult of the relatively good wavelength selectivity of the conventionalcrystal. At the same time, however, the diffraction efficiency of theconventional crystal is lower than that of a HOPG crystal. Thus theradiation detector would only receive a limited proportion of thephotons that actually originated from the element of interest in thesample (see hatched area 1303 in FIG. 13). The small peak 1402 in FIG.14 represents these fluorescent X-rays, which will actually be detectedin this case.

One factor to consider in the design of the X-ray fluorescence analyzeris the possibility to use differently equipped detection channels. Here“differently equipped” means primarily the selection of the diffractorcrystal and the selection of the radiation detector.

FIG. 15 illustrates schematically how an industrial X-ray fluorescenceanalyzer for analyzing samples of slurry may comprise a plurality ofdetection channels. The detection channels are shown in a straight linein FIG. 15 because the representation is schematic. In practice theycould be located for example in a rotationally symmetric manner aroundthe X-ray tube like in FIGS. 4 and 5, each with a field of view directedtowards the slurry handling unit of the X-ray fluorescence analyzer.

The X-ray fluorescence analyzer comprises a first crystal diffractor1501 located in a first direction from said slurry handling unit, saidfirst crystal diffractor 1501 comprising a first crystal. A firstradiation detector 1505 is configured to detect fluorescent X-raysdiffracted by said first crystal 1502 at a first energy resolution. TheX-ray fluorescence analyzer comprises a second crystal diffractor 1511located in a second direction from said slurry handling unit, saidsecond crystal diffractor comprising a second crystal 1512. A secondradiation detector 1515 is configured to detect fluorescent X-raysdiffracted by said second crystal 1512 as a second energy resolution.

As a first assumption it may be assumed that the first crystal 1502 is apyrolytic graphite (HOPG) crystal, and said second crystal 1512 is of amaterial other than pyrolytic graphite, like silicon dioxide, lithiumfluoride, ammonium dihydrogen phosphate, or potassium hydrogenphthalate. Also as a first assumption it may be assumed that the firstand second crystal diffractors are configured to direct to theirrespective radiation detectors characteristic fluorescent radiation of asame element. In other words, the two detection channels are equippeddifferently, but they both aim at detecting the presence andconcentration of the same element in the sample of slurry.

As such, configuring a crystal diffractor to direct to its radiationdetector characteristic fluorescent radiation of a particular element istypically done by 1) selecting a crystal with a particular distancebetween its reticular planes, 2) selecting the curvature of the crystaland the reticular planes, and 3) selecting the angle and distance valuesof the crystal and the slit(s) so that X-rays of just a particularwavelength range will reach the detector, said particular wavelengthrange including the desired characteristic peak of the element ofinterest. The element of interest may have several characteristic peaks,so saying that the two detection channels are configured for measuringcharacteristic fluorescent radiation of the same element does notnecessarily mean that they would be configured for measuring the samecharacteristic peak, although that is not excluded either.

If the two detection channels are configured for measuring the samecharacteristic peak, the measurement results they produce may resemblethose in FIG. 13 (for the channel with the HOPG crystal) and (for thechannel with the other crystal). The task of finding out the actualconcentration of the element of interest may be described in the form ofa method, for example as follows.

The method is aimed at performing X-ray fluorescence analysis, andcomprises irradiating a sample of slurry with incident X-rays andreceiving fluorescent X-rays from the irradiated sample. Due to themeasurement geometry, a first portion of the fluorescent X-rays will bedirected to the first detection channel, and a second portion of thefluorescent X-rays will be directed to the second detection channel. Themethod comprises separating first 1301 and second 1401 predefinedwavelength ranges from respective first and second portions of saidreceived fluorescent X-rays with respective first 1501 and second 1511crystal diffractors. Said first wavelength range 1301 and said secondwavelength range 1401 both include characteristic fluorescent radiationof a same element. Additionally said first wavelength range 1301 is atleast twice as wide as said second wavelength range 1401.

The method comprises detecting the fluorescent X-rays in said first andsecond separated wavelength ranges with respective first 1505 and second1515 radiation detectors. The energy resolution of said first radiationdetector 1505 is better than 300 eV at a reference energy of 5.9 keV.Thus the method comprises producing respective first and seconddetection results. The method comprises calculating a concentration ofsaid element in said sample from at least one of said first and seconddetection results.

Here “at least one” emphasizes the fact that not all detection resultsare best dealt with in equal manner. Very much depends on the sample. Insome samples the concentration of the element of interest may berelatively large, resulting in a relatively large number of detectedfluorescent photons even in the second radiation detector 1515 despitethe modest diffraction efficiency of the second crystal 1512. In someother case the concentration of the element of interest may be so smallthat only a very small and vague peak is visible within the secondwavelength range 1401. In some cases the first wavelength range 1301 mayappears to be relatively clean from any interfering radiation, whilesome other sample may contains significant amounts of some otherelement, the characteristic peak of which is so close that it comesvisible and even dominant in the first wavelength range 1301 but not inthe second wavelength range 1401.

In general the calculating may comprise calculating a combined intensityof background radiation and fluorescent X-rays from others than saidelement using at least one of the first and second detection results.The method may then comprise subtracting, from the total intensitydetected in a wavelength range containing said characteristic peak offluorescent X-rays of an element to be measured in said sample, thecalculated combined intensity of background radiation and fluorescentX-rays from other elements than said element of interest in said sample.The method may then comprise providing the result of said subtracting asthe calculated intensity of said characteristic fluorescent X-ray peak.

The calculating may comprise analyzing from said first and seconddetection results whether the influence of a characteristic peak fromanother element on the first detection result is larger than apredetermined threshold. If said analyzing shows that the influence ofsaid characteristic peak from said other element on the first detectionresult is larger than said predetermined threshold, the method maycomprise calculating said concentration of said element in said samplefrom said second detection result. If, on the other hand, said analyzingshows that the influence of said characteristic peak from said otherelement on the first detection result is not larger than saidpredetermined threshold, the method may comprise calculating saidconcentration of said element in said sample from said first detectionresult.

Another possibility is to form specific models for each measurementchannel per sample line, using calibration samples. The measurementchannel to be used for the actual measurements of that sample line isthen selected on the basis of which of them gives the most accuratecalibration.

The element of interest may be gold, because gold is valuable andbecause reasonably effective methods exist for extracting it even fromflows of slurry where it appears in very low concentrations. There areother elements, interfering characteristic peaks of which may or may notbe present and may appear very close to one of gold. If significantamounts of such interfering elements are present in the sample, thedetection channel with the HOPG crystal may give relatively inaccurateand unreliable results, at least if used alone.

Intermediate forms of these two extreme cases can be presented, in whichthe contribution of the first and second detection results are takeninto account in various ways. The decision about which calculatingmethod is selected can be made for example with an artificialintelligence algorithm that compares the first and second detectionresults to previously obtained comparable results and to some kind ofevaluation data about how the various available calculation methodsperformed with said comparable results.

FIG. 16 illustrates schematically a fluorescent X-ray spectrum thatcomprises two clear peaks 1601 and 1602. In such case the selectedcalculation method may depend on whether the peaks 1601 and 1602 bothare characteristic peaks of the same element of interest, or whether oneof them is a characteristic peak of some interfering element. Thesmaller peaks closer to the energy axis represent the estimateddetection result that a detection channel equipped with a conventional(for example silicon dioxide) crystal would produce of these two peaks.

An interesting case is one where the peaks 1601 and 1602 both are peaksof the element of interest. Particularly interesting is if that one ofthem (here: peak 1601) is more intense, for the measuring of which theSiO2-equipped detection channel is configured. In such a case the bestfeatures of both channels may come into use: the accurate wavelengthselectivity of the silicon dioxide crystal can be used to separate atightly defined wavelength range 1401 that only includes the desiredcharacteristic peak, so that the relatively large intensity of that peakstill gives a sufficient number of counts in the corresponding detectorin a relatively short time. At the same time the good diffractionefficiency of the HOPG crystal can be used to separate a widerwavelength range 1301 that includes the other, lower characteristicpeak. The concentration of the element of interest can be calculatedfrom the detection results given by the two detectors, when the overallperformance of the two detection channels is known from calibrationmeasurements.

A method of the kind described above may be applicable in many caseswhere the characteristic fluorescent radiation comprises a K- or L-peakof an element with 30≤Z≤92, where Z is the atomic number of saidelement. The flexible adaptability of the method suits well formeasuring samples that comprise one or more elements of interest withina matrix consisting of primarily elements with Z≤8, where Z is theatomic number. This is the case of water-based slurries, for example.

The principles that have been discussed above concerning the use of twodetection channels can be generalized to concern the use of three ormore detection channels. The form factor of the detection channel thathas been described above, i.e. the one in which each crystal diffractor601 is enclosed in a casing delimited by a first planar surface 701 anda second planar surface 702 that is parallel to said first planarsurface 701, enables distributing a plurality of detection channels as“cassettes” for example in a rotationally symmetric formation around theX-ray tube. Detection results from detection channels configured todetect characteristic fluorescent radiation of a same element can becombined in various ways as described above. The large number ofdetection channels allows calculating the concentrations of two or moreelements of interest in the sample simultaneously, if the detectionchannels are configured to measure the characteristic fluorescentradiation of such two or more elements of interest. Cross-correlatingthe detection results from channels configured to detect differentelements is also possible. For example if one element has twocharacteristic peaks, one of which is measured with a dedicated firstdetection channel while the other comes close to the characteristic peakof the other element of interest, the detection results from the firstchannel may be used to correct the detection results from that channelthat is configured to measure the characteristic peak of the otherelement.

One factor to consider in the design of an industrial X-ray fluorescenceanalyzer for analyzing samples of slurry is the power of the X-ray tube,and the geometry and dimensioning of the area between the X-ray tube andthe slurry handling unit.

FIG. 17 illustrate the possibility of using so-called transmissiongeometry. The radiation window 203 of an X-ray tube is visible on theright in FIG. 17, and incident X-rays are emitted in the direction ofthe optical axis 204 through a primary filter plate 205. The slurryhandling unit 201 comprises a chamber 1701 with an output slit 1702,from which the sample 202 flows out in a curtain-like form and fallsdownwards under the influence of gravity. The incident X-rays generatefluorescent X-rays in the relatively thin sheet of falling slurry.Reference designator 1703 points at fluorescent X-rays that are directedobliquely backwards, and that can be detected with detection channels(not shown in FIG. 17) placed much like in the geometries describedearlier with reference to FIGS. 2, 3, 4, and 5. Reference designator1704 points at fluorescent X-rays that are directed to other directions,particularly to directions that are on the other side of the sampleflow. These can be detected with detection channels (not shown in FIG.17) placed on that side. This may be a particularly advantageous way ofplacing detection channels, because they can get a better field of viewand consequently a better spatial efficiency of collecting fluorescentX-rays. This may also help to bring the X-ray tube very close to thesample. It has to be noted, though, that proper radiation shieldinggeometrical precautions must be taken in order to prevent any of theincident X-rays from entering the detection channels.

FIG. 18 is a partial cross section of the output portion of an X-raytube 402. The X-ray tube comprises an anode 1801 for generating theincident X-rays. The incident X-rays will be emitted in the direction ofthe optical axis 204 towards the sample 202, which here is shown onlyschematically without the slurry handling unit for reasons of graphicalclarity. It is nevertheless assumed that the slurry handling unit isconfigured to maintain a planar surface 1802 of the sample 202 of slurryon a side facing the X-ray tube 402. As explained earlier, this can beaccomplished for example by providing a sample window with a window foilmade of a material that is transparent to X-rays. The sample window maybe provided in a wall of a sample chamber, for allowing X-rays to passthrough while keeping the sample of slurry within the sample chamber.

Other parts of the X-ray tube that are schematically shown in FIG. 18are the circulation 1803 of cooling water, the ring-shaped cathode 1804for emitting the accelerated electrons, and the radiation window 203.

When the aim is to produce so much fluorescent radiation that even verysmall concentrations of elements of interest could be detected, it isadvantageous if as many photons (of sufficient energy) of the incidentradiation as possible can be made to hit the sample 202. One way ofachieving this is to have a very powerful X-ray tube. According to anembodiment the input power rating of the X-ray tube 402 is at least 400watts. Even more powerful X-ray tubes can be used: according to otherembodiments the input power rating of the X-ray tube 402 may be at least1 kilowatt, or at least 2 kilowatts, or even at least 4 kilowatts. Evenif only a fraction of the power that is announced as the input powerrating of the X-ray tube will eventually come out in the form ofgenerated incident X-rays, the input power rating is nevertheless animportant indicator of the capability of the X-ray tube of producing anintense flux of incident X-rays.

Using X-ray tubes with higher power ratings than earlier means thatradiation shielding must be reconsidered with respect to previouslyknown, lower-powered X-ray sources. According to an embodiment, thickerradiation shielding plates and denser radiation shielding materials maybe used to ensure that ionizing radiation does not leak into areas whereit could be hazardous.

Another way of ensuring a very intense flux of incident X-rays hittingthe sample 202 is to make the distance between the anode 1801 and thesample 202 as small as possible. The slurry handling unit may beconfigured to maintain a shortest linear distance d between the anode1801 and the sample 202, so that d is shorter than 50 mm. In anotherembodiment d may be shorter than 40 mm. In another embodiment d may beshorter than 30 mm.

It must be noted, however, that generally the closer the anode 1801 ofthe X-ray tube 402 is brought to the sample 202, the larger spatialangle around the sample 202 is blocked by the structures of the X-raytube. This is an important factor to consider, because the structures ofthe X-ray tube 402 may block the field of view of the detectionchannels. One way to mitigate this problem is to use an X-ray tube ofthe so-called end window type, and not an X-ray tube of the side windowtype. FIGS. 18 and 19 can be considered to illustrate the use of anX-ray tube of the end window type. In an X-ray tube of this kind theradiation window 203 is generally at one end of a generally tubularstructure, which leaves relatively much free space around said tubularstructure for placing the detection channels. Another possibility wouldbe to use an X-ray tube of the side window type, and to place thedetection channels on one or two sides of the X-ray tube.

In all figures described so far, the optical path between the X-ray tube402 and the sample 202 is also direct, which means that there are nodiffractors therebetween. This is another way of ensuring that a maximumnumber of incident X-ray photons may hit the sample. First, theprovision of a diffractor therebetween would inevitably mean a longerdistance between the anode 1801 and the sample 202, because some spacewould need to be reserved for the diffractor. Second, the mere nature ofa diffractor is to separate only a certain wavelength range from theoriginal radiation spectrum, which would necessarily mean fewer incidentX-ray photons hitting the sample. Other advantageous consequences of notusing any so-called primary diffractor between the X-ray tube 402 andthe sample 202 are the simultaneous provision of incident X-rays forexciting the characteristic peaks of a number of elements in the sampleand that less structural parts are there that could block the field ofview of the detection channels.

In FIG. 18 the optical axis 204 of the X-ray tube 402 is perpendicularagainst the planar surface 1802 of the sample 202. While thisarrangement provides for excellent rotational symmetry for detectionchannels placed around the X-ray tube 402, it is not the onlypossibility. FIG. 19 illustrates an alternative embodiment, in which theoptical axis 204 of the X-ray tube 402 is at an oblique angle againstsaid planar surface. Such an arrangement may help to make the shortestlinear distance d between the anode 1801 and the sample 202 evenshorter, while simultaneously leaving sufficiently free field of viewfor detection channels on at least some sides of the X-ray tube 402.This principle is elaborated upon further in the following withreference to FIGS. 20 and 21.

FIG. 20 shows an X-ray tube 402 and five detection channels seen fromthe direction of the sample. The radiation window 203 of the X-ray tube402 is visible in the middle of the drawing. The entry window of eachdetection channel for receiving fluorescent radiation is located in theproximal end face of the respective crystal diffractor; entry window2001 is shown as an example. For the purpose of making as largeproportion as possible of the generated fluorescent radiation enter adetection channel, it is advantageous to place these entry windows asclose as possible to the sample, and also so that the entry window seesthe sample surface in as large spatial angle as possible. Each of theplurality of crystal diffractors is located at a respective rotationangle around the optical axis of the X-ray tube 402. Each of saidcrystal diffractors is configured to separate a predefined wavelengthrange from fluorescent X-rays that propagate into the respectivedirection, and configured to direct the fluorescent X-rays in therespective separated predefined first wavelength range to a respectiveradiation detector.

FIG. 21 shows an X-ray tube 402 and two detection channels seen from theside. The sample window 301 is schematically shown in FIG. 21: thisillustrates the area where the slurry handling unit is configured tomaintain a planar surface of the sample of slurry on a side facing theX-ray tube 402. Thus this is the area that should be within the field ofview of the X-ray tube 402 in order to make the incident X-rays hit thesample. This illustrates also the area that should cover as largespatial angle as possible in the field of view of the detectionchannels, in order to collect as much fluorescent X-rays as possible.

The optical axis 204 of the X-ray tube 402 is at an oblique angleagainst said planar surface. A first crystal diffractor 1501 is locatedat that rotational angle around said optical axis 204 at which saidplanar surface of said sample covers the largest portion of a field ofview of the first crystal diffractor 1501. Assuming that no otherstructures block any part of the available field of view, in practicethis means that the first crystal diffractor 1501 is located opposite tothe X-ray tube, i.e. in the direction to which an imaginary light beamalong the optical axis 204 would reflect if the sample surface was amirror.

A second crystal diffractor 1511 is located at another rotational anglearound said optical axis 204. In FIG. 21 the second crystal diffractor1511 is located at what could be described as the worst possiblerotational angle, because its view of the sample surface is limited bythat edge of the X-ray tube 402 that comes closes to the sample window301. If said other rotational angle differs by less than 180 degreesfrom that in which the first crystal diffractor 1501 is located, thesecond crystal diffractor 1511 could be located more like one of theplurality of other crystal diffractors in FIG. 20. In such a case theplanar surface of the sample at the sample window 301 would cover aportion of the field of view of the second crystal diffractor 1511 thatwas between the two extremes shown in FIG. 21.

According to an embodiment, the first crystal diffractor 1501 that isplaced at the optimal rotational angle (in terms of field of view) inFIGS. 20 and 21 is the one in which the diffractor crystal is a HOPGcrystal and the radiation detector is a solid-state semiconductordetector. Taken the good diffraction efficiency of the HOPG crystal,such placing of the first crystal diffractor helps to ensure that amaximum number of fluorescent X-ray photons will eventually reach thedetector. If there is some advance knowledge about the assumed levels ofconcentrations of various elements in the samples to be measured, it maybe advantageous to place that crystal diffractor to the most optimalrotational angle that is configured to separate and direct to itsrespective detector the characteristic fluorescent radiation of thatelement of interest that is expected to have the smallestconcentrations.

One factor to consider in the design of an industrial X-ray fluorescenceanalyzer for analyzing samples of slurry is the selection of radiationdetectors in those channels that have diffractor crystals of othermaterials than pyrolytic graphite. The wavelength selectivity ofconventional diffractor crystal materials such as silicon dioxide isrelatively good, which can be interpreted so that there is not as muchneed for accurate energy resolution in the radiation detector as if aHOPG crystal was used. A gas-filled proportional counter may providequite satisfactory detection results in a detection channel that hasother than HOPG as the diffractor crystal, at an advantageously lowermanufacturing cost than a solid-state semiconductor detector.

However, nothing in the foregoing should be interpreted against choosinga solid-state semiconductor detector also for detection channels thathave other than HOPG as the diffractor crystal. Similarly it is not amandatory requirement to use a solid-state semiconductor detector in thedetection channel equipped with a HOPG crystal, if the energy resolutionof another type of radiation detector is found to be sufficient.

FIGS. 22 to 25 illustrate calibration measurements, in which thevertical axis represents concentrations measured with one detectionchannel of a tested apparatus, which was an industrial X-rayfluorescence analyzer for analyzing samples of slurry according to anembodiment. The horizontal axis represents concentrations in the samesamples but measured for prolonged periods with laboratory gradeequipment, in order to as accurate and reliable results as possible. Forthe laboratory measurements the samples of slurry were dried andhomogenized, and the amount of removed water was compensated for throughcalculation, in order to make the laboratory measurements comparablewith the industrial-type measurements. If a calibration measurement ofthis kind shows the points settling along a straight line, the twodifferent apparatuses give highly matching results, which means that thetested apparatus is very accurate. Deviations from a straight line showthat the tested apparatus produces inaccurate results.

The scales are arbitrary, but the scales in FIGS. 22 and 23 are thesame, and the scales in FIGS. 24 and 25 are the same. The element ofinterest was gold in all measurements. FIGS. 22 and 23 representmeasurements of samples in which the concentration of an interferingelement was below 300 ppm, while the measurements of FIGS. 24 and 25 itsconcentration varied between 0 and 2%. An interfering element is onethat has a characteristic fluorescent peak close to at least onecharacteristic fluorescent peak of the element of interest.

FIGS. 22 and 24 represent cases in which the measurement with the testedapparatus was made using a detection channel that had a silicon dioxidecrystal in the crystal diffractor and a gas-filled proportional counteras the radiation detector. FIGS. 23 and 25 represent cases in which themeasurement with the tested apparatus was made using a detection channelthat had a HOPG crystal in the crystal diffractor and a solid-statesemiconductor detector as the radiation detector.

A comparison of FIGS. 22 and 23 shows that when the concentration of aninterfering element is small, the detection channel with a HOPG crystaland a solid-state semiconductor detector gives more accurate detectionresults than the detection channel with a silicon dioxide crystal and agas-filled proportional counter. The average error betweenconcentrations measured with the HOPG channel of the tested apparatusand those measured in laboratory was +/−0.24 ppm, while the comparableaverage error with a silicon dioxide crystal and a gas-filledproportional counter was +/−0.56 ppm.

A comparison of FIGS. 24 and 25 shows that when the concentration of aninterfering element is significant, the detection channel with a HOPGcrystal and a solid-state semiconductor detector gives less accuratedetection results than the detection channel with a silicon dioxidecrystal and a gas-filled proportional counter. The average error betweenconcentrations measured with the HOPG channel of the tested apparatusand those measured in laboratory was +/−1.62 ppm, while the comparableaverage error with a silicon dioxide crystal and a gas-filledproportional counter was +/−0.42 ppm.

The results shown in FIGS. 22 to 25 can be utilized in many ways. Forexample, the industrial X-ray fluorescence analyzer for analyzingsamples of slurry may be equipped with first, second, and thirddetection channels, of which the first and second detection channels areboth equipped with crystal diffractors configured to separate and directto their respective detectors characteristic fluorescent X-rays of thesame element, like gold. The first detection channel may be one with aHOPG crystal and a solid-state semiconductor detector, and the seconddetection channel may be one with a silicon dioxide crystal and agas-filled proportional counter. The third detection channel may beequipped with a crystal diffractor configured to separate and direct toits respective detector characteristic fluorescent X-rays of aninterfering element. The detection results of all three detectionchannels can be then analyzed. If the detection results produced by thethird detection channel show there to be a significant concentration ofthe interfering element in the sample, the calculation of theconcentration of gold would emphasize more (or even use exclusively) thedetection results of the second detection channel. Correspondingly ifthe detection results produced by the third detection channel show thereto be only an insignificant concentration of the interfering element inthe sample, the calculation of the concentration of gold would emphasizemore (or even use exclusively) the detection results of the firstdetection channel.

Many advantageous features of the industrial X-ray fluorescence analyzerfor analyzing samples of slurry have been described above. In the endthey all serve a common purpose, which is to make reliable measurementsof even very small concentrations of elements of interest in slurries ofvarious kinds, at reasonable cost and under the harsh conditions that anindustrial environment may place: short measurement times; extremetemperatures; frequent occurrence of humidity, dust, and dirt; longintervals between servicing; and the like. The advantageous features maybe combined with each other in numerous ways, so that the mostadvantageous combination may depend on a particular case and itsspecific boundary conditions.

It is obvious to a person skilled in the art that with the advancementof technology, the basic idea of the invention may be implemented invarious ways. The invention and its embodiments are thus not limited tothe examples described above, instead they may vary within the scope ofthe claims. As an example, even of gold has been frequently mentionedabove as a typical element of interest, the same principles areapplicable also to measurements of other elements of interest. Examplesof such other elements of interest are for example copper, silver,metals of the platinum group, and uranium.

1-25. (canceled)
 26. An X-ray fluorescence analyzer, comprising: anX-ray tube for emitting incident X-rays in the direction of a firstoptical axis, a slurry handling unit configured to maintain a constantdistance between a sample of slurry and said X-ray tube, a first crystaldiffractor located in a first direction from said slurry handling unit,said first crystal diffractor comprising a first crystal, a firstradiation detector configured to detect fluorescent X-rays diffracted bysaid first crystal at a first energy resolution, a second crystaldiffractor located in a second direction from said slurry handling unit,said second crystal diffractor comprising a second crystal, a secondradiation detector configured to detect fluorescent X-rays diffracted bysaid second crystal at a second energy resolution, characterized inthat: said first crystal is a pyrolytic graphite crystal, said secondcrystal is of a material other than pyrolytic graphite, and said firstand second crystal diffractors are con-figured to direct to theirrespective radiation detectors characteristic fluorescent radiation of asame element.
 27. The X-ray fluorescence analyzer according to claim 26,wherein said second crystal is one of: a silicon dioxide crystal, alithium fluoride crystal, an ammonium dihydrogen phosphate crystal, apotassium hydrogen phthalate crystal.
 28. The X-ray fluorescenceanalyzer according to claim 26, wherein said first energy resolution isbetter than 300 eV at a reference energy of 5.9 keV.
 29. The X-rayfluorescence analyzer according to claim 26, wherein said firstradiation detector is one of: a PIN diode detector, a silicon driftdetector, a germanium detector, a germanium drift detector.
 30. TheX-ray fluorescence analyzer according to claim 26, wherein said secondradiation detector is a gas-filled proportional counter.
 31. The X-rayfluorescence analyzer according to claim 26, wherein said element isgold.
 32. The X-ray fluorescence analyzer according to claim 26,wherein: said slurry handling unit is configured to maintain a planarsurface of said sample of slurry on a side facing said X-ray tube, saidfirst optical axis is at an oblique angle against said planar surface,said first crystal diffractor is located at that rotational angle aroundsaid first optical axis at which said planar surface of said samplecovers the largest portion of a field of view of the first crystaldiffractor, and said second crystal diffractor is located at anotherrotational angle around said first optical axis.
 33. The X-rayfluorescence analyzer according to claim 26, wherein: said slurryhandling unit is configured to maintain a planar surface of said sampleof slurry on a side facing said X-ray tube, and said first optical axisis perpendicular against said planar surface.
 34. The X-ray fluorescenceanalyzer according to claim 26, wherein the input power rating of saidX-ray tube is at least 400 watts.
 35. The X-ray fluorescence analyzeraccording to claim 34, wherein the input power rating of said X-ray tubeis at least 1 kilowatt, preferably at least 2 kilowatts, and morepreferably at least 4 kilowatts.
 36. The X-ray fluorescence analyzeraccording to claim 26, wherein the optical path between said X-ray tubeand said slurry handling unit is direct with no diffractor therebetween.37. The X-ray fluorescence analyzer according to claim 26, wherein theX-ray tube comprises an anode for generating said incident X-rays, andsaid slurry handling unit is configured to maintain a shortest lineardistance that is shorter than 50 mm, preferably shorter than 40 mm, andmore preferably shorter than 30 mm between said sample of slurry andsaid anode.
 38. The X-ray fluorescence analyzer according to claim 37,wherein said X-ray tube is an X-ray tube of the end window type.
 39. TheX-ray fluorescence analyzer according to claim 26, wherein thediffractive surface of said pyrolytic graphite crystal is one of thefollowing: a simply connected surface curved in one direction; a simplyconnected surface curved in two directions; a rotationally symmetricsurface that is not simply connected.
 40. The X-ray fluorescenceanalyzer according to claim 26, further comprising: an analyzer body, afront wall of said analyzer body, an opening in said front wall, and aholder for removably holding said slurry handling unit against an outerside of said front wall and aligned with said opening in said frontwall.
 41. The X-ray fluorescence analyzer according to claim 40, whereinsaid X-ray tube and said first crystal diffractor are both inside saidanalyzer body, on the same side of said front wall.
 42. The X-rayfluorescence analyzer according to claim 26, comprising a filter plateon the optical path between said X-ray tube and said slurry handlingunit.
 43. The X-ray fluorescence analyzer according to claim 42, whereinsaid filter plate is located closer to said X-ray tube than to saidslurry handling unit.
 44. The X-ray fluorescence analyzer according toclaim 26, comprising a calibrator plate and an actuator configured tocontrollably move said calibrator plate between at least two positions,of which a first position is not on the path of the incident X-rays anda second position is on the path of the incident X-rays and in a fieldof view of the first crystal diffractor.
 45. A method for performingX-ray fluorescence analysis, comprising: irradiating a sample of slurrywith incident X-rays and receiving fluorescent X-rays from theirradiated sample, separating first and second predefined wavelengthranges from respective first and second portions of said receivedfluorescent X-rays with respective first and second crystal diffractors,wherein said first wavelength range and said second wavelength rangeboth include characteristic fluorescent radiation of a same element, andwherein said first wavelength range is at least twice as wide as saidsecond wavelength range, detecting the fluorescent X-rays in said firstand second separated wavelength ranges with respective first and secondradiation detectors, wherein the energy resolution of said firstradiation detector is better than 300 eV at a reference energy of 5.9keV, thus producing respective first and second detection results, andcalculating a concentration of said element in said sample from at leastone of said first and second detection results.
 46. The method accordingto claim 45, wherein said calculating comprises: calculating a combinedintensity of background radiation and fluorescent X-rays from othersthan said element using at least one of the first and second detectionresults, subtracting, from the total intensity detected in a wavelengthrange containing said characteristic peak of fluorescent X-rays of anelement to be measured in said sample, the calculated combined intensityof background radiation and fluorescent X-rays from other elements thansaid element of interest in said sample, and providing the result ofsaid subtracting as the calculated intensity of said characteristicfluorescent X-ray peak.
 47. The method according to claim 45, whereinsaid calculating comprises: analyzing from said first and seconddetection results whether the influence of a characteristic peak fromanother element on the first detection result is larger than apredetermined threshold, if said analyzing shows that the influence ofsaid characteristic peak from said other element on the first detectionresult is larger than said predetermined threshold, calculating saidconcentration of said element in said sample from said second detectionresult, and if said analyzing shows that the influence of saidcharacteristic peak from said other element on the first detectionresult is not larger than said pre-determined threshold, calculatingsaid concentration of said element in said sample from said firstdetection result.
 48. The method according to claim 45, wherein saidelement is gold.
 49. The method according to claim 45, wherein saidcharacteristic fluorescent radiation comprises a K- or L-peak of anelement with 30≤Z≤92, where Z is the atomic number of said element. 50.A method according to claim 45, wherein said sample comprises saidelement within a matrix consisting of primarily elements with Z≤8, whereZ is the atomic number.